In a large mixing bowl, combine the 4 egg yolks, 2 whole eggs, and ¾ cup of sugar. Whisk these ingredients together until they're well blended. The mixture should take on a light, creamy color.

In a small saucepan, add the 2 cups of citrus juice. Heat over medium-high until it comes to a gentle boil.

Once the citrus juice is boiling, remove the saucepan from the heat. Add the zest from 1 lemon to the hot citrus juice, stirring to incorporate.

Begin to slowly pour the hot citrus juice into your egg and sugar mixture while continuously whisking. This process is called tempering. It helps prevent the eggs from curdling when they come into contact with the hot liquid.

After you've combined the citrus juice with the eggs and sugar, pour the mixture back into your saucepan.

At this point, it's a good idea to prepare a clean bowl with a strainer placed on top, positioned near your stove for easy access.

Return the saucepan to the stove and heat the mixture over medium heat, stirring frequently. Use a kitchen thermometer to monitor the temperature. Once the mixture reaches 180°F (82°C) and has thickened to a custard-like consistency, remove from heat. This can take around 10 minutes, but timing can vary.

Immediately pour the mixture through the strainer and into the prepared bowl. This helps remove any bits of cooked egg, resulting in a silky smooth curd.

After straining, add the ½ cup of cubed unsalted butter and ¼ teaspoon of salt to the mixture. Stir well until the butter is fully melted and incorporated.

Once cooled, transfer the citrus curd to an airtight container and store in the refrigerator. The curd will continue to thicken as it cools.